Vessel comprising transverse skirts

ABSTRACT

The invention relates to a vessel having an elongated shape, and being moored to the sea bed in a substantially fixed orientation. The vessel comprises at least along its longitudinal sides, near keel level, two transverse skirts having such a width that the natural roll period of the vessel is above a predetermined period, the natural roll period of the vessel without the skirts being below said predetermined period.

[0001] The invention relates to a vessel having an elongated shape, and being moored to the seabed in a substantially fixed orientation.

[0002] It is known that elongated vessels, in particular vessels having a length-to-width ratio of 1,5 or less, may be subject to roll instability or Mathieu instability. This roll instability, or parametric resonance causes roll due to heave-induced modulation of the roll hydrostatic stiffless of the system. In particular, the most severe condition would occur when the heave natural period is half the roll natural period.

[0003] It is an object of the present invention to design an elongated vessel, preferably of substantially rectangular shape, with improved stability. It is a further object of the present invention to design an elongated vessel in which roll motions are damped and in which the natural roll period is increased.

[0004] It is another object of the present invention to provide a vessel that can be built under ship construction conditions rather than under complex offshore conditions.

[0005] Hereto the vessel according to the invention comprises at least along its longitudinal sides, near keel level, two transverse skirts having such a length and width that the natural roll period of the vessel is above a predetermined period, the natural roll period of the vessel without the skirts being below said predetermined period.

[0006] It was found that the addition of skirts over the length of the elongated vessel slows the vessel in roll, in view of the added mass, such that the inertia of the vessel for roll motions is increased. Therefore it is possible to construct the vessel such that the natural roll period is above the roll period of severe storm conditions. For example, the natural roll period of the vessel without the skirts may be about 10-18 s. By providing the longitudinal skirts, the natural roll period may be increased to 20-25 s, which is above the wave period of 15-16 s of a severe storm. The slower roll motions increase the comfort for personnel on the vessel and reduce fatigue in the mooring lines and production systems (for instance fewer problems with separators).

[0007] By providing the skirts of the present invention, the roll response of the vessel at wave periods can be significantly reduced, however a larger roll response is shifted to the vessel's own natural roll period.

[0008] In one embodiment, the length-to-width ratio of the vessel is at least 1,5, preferably at least 2, the skirts having a width of at least 5% of the width of the vessel near keel level, and extending over at least 60% of the length of the vessel, preferably at least 90%, most preferably over at least substantially the whole length.

[0009] By providing the skirts along the majority of the length of the vessel, a large increase in roll stability, and a very favorable lengthening of the natural roll period and damping of roll motions is achieved. Preferably the skirts extend along the longitudinal and along the short sides of the vessel. Mooring lines may be attached to the skirts, and decks structures may be supported on the skirts.

[0010] It is preferred that the draft of the vessel is below 30 m, such that it can be built on shore, under regular ship building conditions. Furthermore, it was found that vessels having a relatively low draft level, in combination with the skirts of the present invention, are very stable.

[0011] Preferably a vessel comprises a central well and at least one riser and a drill string extending from a sub sea structure through said well to a supporting structure on the vessel, such as a drilling rig. The drill string and/or riser are suspended from a pivotable support structure above the well such as a tension leg deck which is described in EP-A-1.036.914 “Method for installing a number of risers or tendons and vessel for carrying out said method” in the name of the applicant. The Tension Leg Deck (pivotable support) results in advantages such as a more stable vessel and drilling situation, so that longer drilling times are possible (less downtime). In case of LNG production, the pivotable support structure results in less sludging and safer working conditions.

[0012] It is known from WO99/44882 to provide corrugated keels along the bilge of an FPSO with weathervanig capacity. The bilge keels are relatively narrow and help in generating vorticity such that roll motions are damped. The natural roll period is not altered by the bilge keels.

[0013] GB 2 243 118 discloses a semi-submersible vessel of generally circular symmetry with a peripheral skirt. The semi-submersible vessel has a relatively large draft and needs to be built according to offshore construction standards.

[0014] In WO98/0439 an elongated vessel is disclosed having lips extending along the whole length of the vessel, for damping of the heave motions. Roll motions of the known vessel are avoided by aligning the vessel, which is provided with a dynamic positioning system, with the wind and wave directions. In contrast thereto, the vessel according to the present invention is moored which could be spread moored, a semi-weathervaning mooring system or a turret mooring system, and could be subject to beam waves, perpendicular to its length direction.

[0015] In OMAE 1996, Volume I-Part B, Offshore Technology: Design and hydrodynamic performance analyses of a large simple barge, it is described that large skirts of a symmetrically shaped BARBOX (square barge) increase the heave added mass, and thereby provide a heave damping. It was observed, however that the freeboard of the skirted option was considered adequate, the roll angles observed were excessive. In contrast thereto, the roll stability of the elongated vessel of the present invention is increased by the presence of longitudinal skirts. Also the known square barge has a draft of 32 m, such that it is unsuitable to be constructed in a regular shipyard, but must be assembled offshore.

[0016] In Norway offshore, there is a picture of Akers Maritime's Buoyform concept, as described in detail on page 114, and describing a circular cone shaped SPAR with skirts. Mathieu instabilities of such a buoy have caused pitch/roll motions in the order of 30-40 degrees, where motions of only 1-3 degrees were expected.

[0017] IN OTC 10-953 “Alternative Shape of Spar Platforms for use in Hostile Areas”, alternative hull shapes are shown in FIG. 10 on page 6. The Mathieu instability is explained and in FIG. 17 it is mentioned the unstable heave/pitch motion is so large that the deck hits the water surface.

[0018] The invention will be explained in detail with reference to the accompanying drawing. In the drawing:

[0019]FIG. 1 shows a frontal view of a drilling and production barge having skirts according to the invention; and

[0020]FIG. 2 shows a side view of a drilling and production barge haying skirts according to the invention.

[0021]FIG. 1. shows a barge 1 having a length W1 of 160 m, the longitudinal skirts 2, 3 having a width of 15 m each, such that the total length of the barge at the skirts, W2, is 190 m. The height h1 of the skirts 2,3 is 6 m, the draft level h2 being 22 m, the height of deck level h3 being 32 m. The high volume of the skirts add a large inertia to the roll motion of the vessel because of the large amount of water displacement upon roll, hence providing a larger natural roll period.

[0022] On the barge 1 a drilling rig 5 is placed on a rig support structure 6. From the rig 5 a drill string 7 extends towards the sea bed 33. The drill string 7 and production risers 8, 9 are suspended from a deck 10. The deck 10 also supports a blow-out preventor 11. The deck is pivotably suspended from cables 13, 14, which run along sheaves 15, 16. The cables 13, 14 carry counter weights 17, 18 below water level. The drill string 7 and risers 8, 9 extend through a central well 20 in the barge 1 in which the deck 10 is suspended. The cables 13, 14 extend through tapering shafts 21, 22 through the height of the vessel 1. The vessel carries on its deck an oil separation plant 25, a gas compression plant 26, a drilling equipment compartment 27, utility space 28 and a power generator 29. Anchor lines 30, 31 moor the vessel 1 to the sea bed 33. As can be seen in FIG. 2, the skirts 2′, 3′, support part of the weight of deck structures such as accommodation 35 or a drilling pipe and casing reck 36 via columns 39, 40 which rest on the skirts 2′, 3′. In this way the skirts provide for additional deck space of the barge.

[0023] As is evident from FIG. 2, the width W3 of the barge is 60 m, and the total width, W4, including the skirts 2′, 3′, is 90 m. The skirts 2, 2′, 3, 3′, extend around the vessel, on all sides. 

1. Vessel having an elongated shape, and being moored to the sea bed in a substantially fixed orientation, the vessel comprising at least along its longitudinal sides, near keel level, two transverse skirts having such a width that the natural roll period of the vessel is above a predetermined period, the natural roll period of the vessel without the skirts being below said predetermined period.
 2. Vessel according to claim 1, wherein the predetermined period is at least 18 s, preferably at least 20 s.
 3. Vessel according to claim 1 or 2 where the maximum roll response angle of the vessel with skirts is larger at the vessel's natural roll period than at the period of the waves which also cause a roll response at the wave period.
 4. Vessel according to claim 1, 2 or 3 wherein the volume of a skirt is least 2% of the total volume of the barge.
 5. Vessel according to any of claims 1-4, wherein the roll instability of the vessel with the skirts is significantly reduced compared to the roll instability of the vessel without the skirts.
 6. Vessel according to any of claims 1-5, wherein the length-to-width ratio of the vessel is at least 1,5, preferably at least 2, the skirts having a width of at least 5% of the width of the vessel near keel level, and extending over at least 60% of the length of the vessel, preferably at least 90%, most preferably over at least substantially the whole length.
 7. Vessel according to any of the preceding claims, the vessel comprising skirts along its transverse sides.
 8. Vessel according to any of the preceding claims, the draft of the vessel being lower than 30 m.
 9. Vessel according to any of the preceding claims, the skirts being substantially horizontal.
 10. Vessel according to any of the preceding claims, the skirts being of a generally box-like construction having a width of at least 3 m and a height of at least 3 m.
 11. Vessel according to any of the preceding claims, the skirts being perforated.
 12. Vessel according to any of the preceding claims, where the vessel is moored to the seabed with mooring lines.
 13. Vessel according to any of the preceding claims, the skirts comprising attachment means, the anchor lines being attached to the attachment means.
 14. Vessel according to any of the preceding claims, the vessel comprising a central well, and at least one riser and/or drill string extending from a sub sea structure, through said well to a supporting structure on the vessel.
 15. Vessel according to claim 14, the risers and/or drill string being suspended from the supporting structure above the well, the supporting structure being suspended from suspension means, such that it can be placed at an angle with respect to a horizontal plane of the vessel.
 16. Vessel according to any of the preceding claims, wherein a deck structure is supported by columns resting on the skirts. 